Field of the Invention
The present invention relates to arrangements for performing inspections of internal parts of a cryostat, to observe ice build-up within sensitive parts of the cryostat and to prompt preventative maintenance procedures in response to detection of ice.
Description of the Prior Art
FIG. 1 shows a conventional arrangement of a cryostat including a cryogen vessel 12. A cooled superconducting magnet 10 is provided within cryogen vessel 12, itself retained within an outer vacuum chamber (OVC) 14. One or more thermal radiation shields 16 are provided in the vacuum space between the cryogen vessel 12 and the outer vacuum chamber 14. In some known arrangements, a refrigerator 17 is mounted in a refrigerator sock 15 located in a turret 18 provided for the purpose, towards the side of the cryostat. Alternatively, a refrigerator 17 may be located within access turret 19, which retains access neck (vent tube) 20 mounted at the top of the cryostat. The refrigerator 17 provides active refrigeration to cool cryogen gas within the cryogen vessel 12, in some arrangements by recondensing it into a liquid. The refrigerator 17 may also serve to cool the radiation shield 16. As illustrated in FIG. 1, the refrigerator 17 may be a two-stage refrigerator. A first cooling stage is thermally linked to the radiation shield 16, and provides cooling to a first temperature, typically in the region of 80-100K. A second cooling stage provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K.
A negative electrical connection 21a is usually provided to the magnet 10 through the body of the cryostat. A positive electrical connection 21 is usually provided by a conductor passing through the vent tube 20.
For fixed current lead (FCL) designs, a separate vent path (auxiliary vent) (not shown in FIG. 1) is provided as a fail-safe vent in case of blockage of the vent tube 20.
MRI magnets in the field must be checked for any air ingress which results in ice building up inside the cryostat. The ice can build-up over a period of time. There are no set times for ice build-up to occur, and any given cryostat may not ice at all. Any ice in the cryostat is a problem and can cause problems with helium re-condensing, helium filling, venting, and quenching.
Existing equipment and methods for detecting and removing ice build-up are complex and difficult. Such checks are accordingly carried out only rarely. Icing inside the cryostat is usually found only when a magnet is serviced or has a problem for any reason, typically after a significant build-up of ice has taken place. Removal of such a large ice build-up requires significant down time and expense to correct. The necessary equipment used by an engineer is bulky and parts of the equipment are damaged easily by the cryogenic temperatures. The equipment requires the magnet to be ramped down to zero field so it can be used safely, this requires a power supply to be despatched to the site.
Known disadvantages of the conventional arrangements for checking for ice build-up include the following:
Detection only occurs once the ice has built up to such a level that it has become a problem.
If ice build-up is suspected, special bore scope equipment and power supplies must be shipped to the magnet, wherever it may be in the world, which is time consuming, complex and costly. The equipment itself is bulky and costly and so is not provided to local service technicians as part of a routine servicing kit. A specially trained service technician is required to undertake the check, which may involve significant travel and expense.
The bore scope equipment has a short life when used at cryogenic temperatures, and must be replaced frequently, although it is expensive.
Conventional methods for checking for ice build-up require that the magnet is at zero field. The equipment required for ramping a magnet down to zero field, then back up to full field after checking is costly and bulky. Such ramping down and ramping up also consumes significant amounts of cryogen.